The efficient coupling of single optical fibers or fiber optic bundles has, in the past, required termination of the optical fiber in some manner, as with a ferrule, so that the optical fiber can be precisely centered and aligned with an opposing ferruled optical fiber. The coupling system for the fibers must preferably be low loss and provide for easy termination while at the same time being inexpensive. Low loss implies less than 1 dB insertion loss, whereas ease of termination refers to termination without the use of epoxy, polishing or the addition of index matching fluid.
One of the many methods of terminating an optical fiber includes the utilization of cylindrical rods which are positioned in a cluster so as to surround the end of an optical fiber. The coaction of the outer surfaces of the rods, when mounted in a cluster about an optical fiber, provides for the centering of the fiber in the interstitial space between the rods, with the outer surfaces of the rods both providing for the centering of the fiber in the interstitial space and the location of the fiber with respect to the centerline of a ferrule in which the rods are mounted.
One such rod connector is illustrated in U.S. Pat. No. 4,183,619, which utilizes four equal diameter metal rods or pins to locate a fiber concentric with the outer dimension of the ferrule body into which the pins are inserted. The rigid nature of the pins and the tolerance variations in the diameter of optical fibers require that the interstitial space of the pin cluster be as large as the largest expected fiber. Thus, fibers with smaller diameters will not be accurately centered because of the clearance between the fiber and the rigid pins or rods.
U.S. Pat. No. 4,056,305 illustrates the use of resilient rods to surmount the fiber diameter variation problem. In this patent, each set of three rods contains a fiber which becomes axially aligned with its mate when the rod sets are inserted into a centrally bored elastomeric alignment element or sleeve. The resilience of the rods, when compressed by the walls of the bore, accommodates diameter variations of the fibers. However, the connection system of this patent has the drawback of not gripping the fiber in an unmated condition. That is, when the rod clusters are removed from the alignment sleeve, the rods no longer grip the fiber, allowing axial movement. Moreover since ferrules are not utilized, it is difficult to control the end separation between the ends of the opposed fibers in the alignment sleeve. Note also U.S. Pat. No. 4,050,781 in which three rods are located inside a flexible sheath.
A flat cable connector which utilizes rod clusters is illustrated in U.S. Pat. No. 4,201,444, in which a pair of rod clusters, each involving three equal diameter rods are mounted in a V slot provided in a block of material. The optical fibers are inserted into the interstitial space between the three rods of each cluster and once appropriately positioned, a cover compresses the rods slightly so as to retain the clusters and fibers in their previously aligned positions. Moreover, an embodiment is shown in which a unitary three rod cluster is molded to a centrally-apertured base with the rods sprung outwardly. This three cluster is forced rearwardly into a flexible or elastic sleeve with an optical fiber in the interstitial space between the outwardly sprung rods. The unitary rod cluster is difficult to mold because the rods must be sprung-apart, because the lead in hole is difficult to align and because flashing is likely to occur at the lead in hole. Furthermore this lead in hole does not angularly align the fiber parallel with the rods. Also, proper end separation of fibers is difficult to achieve because the fiber is loose in an unmated condition. Repeated opening of this type connector opens each rod cluster allowing contaminating particles to enter and causing transverse misalignment. Finally, providing an elastic or flexible sleeve with a concentric bore is difficult. If the bore is not concentric, the centerline of the rod cluster will be displaced with respect to the outer dimension of the sleeve, thereby precluding accurate coaxial alignment with respect to the outer dimension of the sleeve. Note also the flat cable connectors of U.S. Pat. Nos. 4,088,386 and 4,102,561, in which vee-shaped slots are used to align fibers.
Aside from the above-noted difficulties with respect to the prior art rod termination systems, the primary problem associated with each of these systems is the utilization of rods having a single diameter. The difficulty in using rods of a single diameter is that they are very difficult to mount in a connector. Either the individual unsupported rods must be simultaneously forced into a sleeve with a fiber therebetween, or they must be molded with a base and sprung or opened outwardly to provide space through which the optical fiber may pass.